"It's with mixed emotions that I move on," Zerhouni, 57, said in a press conference today, according to The Scientist, adding that he doesn't "have a job lined up."

Though he opposed limits on federally funded stem-cell research, it wasn’t until last year that Zerhouni split with President Bush and told Congress that the Bush administration ban on research that used embryonic lines created after Aug. 9, 2001 should be lifted.

"It is clear today that American science will be better served and the nation would be better served if we let our scientists have access to more cell lines," Zerhouni said then. "It is very clear from my point of view that the current cell lines will not be sufficient to do the research we want to do…. It's not possible for me to see how we can continue the momentum of science and in stem cell research with the lines we currently have."